Telephone communicating system



Dec. 3, 1935. A. J. SORENSEN ET AL 2,022,738

TELEPHONE COMMUNICATING SYSTEM Original Filed Jan. 28, 1933 IN VENT 0R5 Andrew J (So/mason am: y Leland D. W/zz'tolook 623ml THEIR Ew E 3 .EQ ENQ km N ATTORNEY Patented Dec. 3, 1935 TELEPHONE COMMUNICATING SYSTEM Andrew J. Sorensen, Edgewood, and Leland D. Whitelock, Wilkinsburg, Pa., assignors to The Union Switch & Signal Company, Swis'svale,

'- Pa., a corporation of Pennsylvania Application January 28, 1933, Serial No. 654,004

Renewed June 9, 1934 19 Claims. (01.1%9-82) or between two trains, or between two locations on the same train, over a communicating circuit including the traffic rails such, for example, as disclosed in the L. O. Grondahl application Serial No. 450,135, filed May 6, 1930, for Electric train signaling systems. For such telephone systems, it has been proposed to modulate a carrier frequency current at voice frequencies and then to suppress the carrier and one side band, supplying the communicating circuit with the remaining side band only. At the receiving location, the transmitted sideband is first combined with a local carrier and demodulated; and the resulting audio frequencies are then amplified and supplied to a telephone receiver or loud speaker. In such systems the outgoing current is amplified to a relatively high energy level, and'the receiv-' ing apparatus includes an, amplifier of high gain. To provide the necessary two-way operation, separate receiving and transmitting equipments are installed at eachlocation, together with switching apparatus to selectively transfer the .communicating circuit from one equipment to the other. The switching apparatus set forth in our present application to selectively transfer the communicating circuit between the transmitting and the receiving equipments is somewhat similar to the switching apparatus set forth in the copending applicationior Communication systems, Serial No. 729,806, filed June 9, 1934, by Leland D. Whitelock et al., and which pending ap plication contains claims directed to such switching apparatus.

A feature of our invention is the provision of new and novel apparatus for ,a carrier current telephone system wherewith both the receiving The accompanying drawing" is a diagrammatic viewof one form or apparatus embodying our invention, and this apparatus is particularly adapted to.telephonesystems for railway trains. It will be understood, however, that we do not limit. ourselves to telephone systems for railway trains, 5 butour invention is equally useful for other carrier systems of telephoning. The form of apparatus here disclosed will serve to illustrate the different forms of apparatus embodying our invention;

In the accompanying drawing, the reference characters 2 and 3 designate inductor coils mounted on a railway train in inductive relation with the tramc rails l and l, respectively; To clarify the following description, it will be con- 15 sidered that telephone communication between the locomotive and the caboose of a freight train is to be provided, and hence, both the locomotive and the caboose will be equipped with the apparatus here disclosed, and a pair of inductor coils 20 2 and 3 will be mounted both on the locomotive and on the caboose. These inductor, coils 2 and 3 are adapted. to be-connected to the input of a receiver AR, or to the output of a transmitter AT, by means of a manually operated circuit 25 controller 4. With the circuit controller 4 in engagement with a contact 5, the coils 2 and 3 are connected to the receiver AR by a circuit that extends from the right-hand terminal of the coil 3 over wire 6, contact 4-5, filter Fl, tuning con- 30 denser l, and wire 8 to the left-hand terminal of coil 2, the two coils 2 and 3 being connected in 'series. The filter, Fl is connected to the input of an amplifier AM by wires l0 and II, and the output of the amplifier AM is connected over 35 wires l3 and I4 to the primary winding l2 of a coupling transformer T2 to be referred to later. The filter Fl is preferably a band-pass filter so proportioned and adjusted as to pass a carrier current side band having a width equal to the :10

voice frequency range. The amplifier AM may be any one of several types adapted to amplify the side band frequencies passed to its input by the filter. Fl. The specific type of neither the filter Fl, nor of the amplifier AM forms a part of 45 our invention, and these devices are shown can ventionally only in order to simplify the drawing.

Raising the circuit controller 4 into engage- "mnt with a contact l5 connects the inductor coils 2 and 3 with the output of the transmitter 50 AT, the circuit extending from the right-hand terminal of coil 3, along wire 6, contact 4-l5 tuning condenser l6, secondary winding ll of an output transformer Tlll of transmitter AT,

and wires l8 and't to the left-hand terminal of 5;

coil 2. It follows that in the lower or normal position of the circuit controller 4,- where it is in engagement with contact 5, the communicating circuit is coupled with the receiver AR through the medium of the coils 2 and 3; and in the .ll per or" reverse position of the circuit controller 4, where it engages the contact I5, the

'- coils 2 and 3 form a coupling between the tramc rails I and I of the communicating circuit and the transmitter AT.

It has been proposed for train telephone'systems to carry the microphone, designated in the drawing by the reference character M, in a rack, not shown, where it is normally inactive. The microphone is rendered active to transmit a telephone message by the'operator moving a handle I9 to the right in the drawing, to a position indicated by a dotted line. As shown schematically,

\the circuit-controller 4 and a second circuit controller 20 are both connected to and operated 'by the handle I9. Moving handleIS te'the right operates the circuit controller 4 from engagement with contact 5 into engagement with'contact I5, and also operates the controller 23 into engagement with a contact 2|. Moving the handle I9 from the dotted line position back to the position shown by the solid'lines in the drawing, operates the controllers 4 and 20 to the positions shown in the drawing. As pointed out above, the normal position of controller 4 connects the coils 2 and 3 to the receiver AR, and the reverse position of controller 4 transfers the coils 2 and3 to the transmitter AT. The reverse position of controller 20 makes engagement with the contact 2| and completes a circuit easily traced for energizinga series of relays 22, 23, 24, 25 and 25 in parallel. It follows that as long as the microphone M is inactive and the handle I3 is in its normal position, the trailic rails I and I are coupled to the receiver AR, and the relays 22, 23, 24, 25 and 26 are deenerglzed; and the moving of the handle I9 preparatory to rendering the microphone active couples the 'trafllc rails I and I with the transmitter AT, and energizes the above series of relays.

A first stage amplifier, designated as a whole by the reference character AMI, is provided to amplify the voice frequencies of a telephone mesback contact 32' closed, the grid 29 is connected to the secondary winding 33 of the coupling transformer T2, a biasing element comprising a resistor 34 and a condenser 35 in parallel being inserted in the connection between the opposite terminal of the secondary winding 33 and the filament 30. Energizing relay 22 to close its front contact 36 transfers the grid 29 to a circuit that includes the secondary winding 31 of a coupling transformer TI as will be understood by an inspection of the drawing. The primary 38 of transformer TI is connected to the micro-' phone M through a battery 39, and transformer TI is so proportioned as to match the impedance of the microphone to the grid impedance of tube 21.

The plate 28 of tube 21 is normally connected filament 52 of tube 5|.

to the, primary winding 43 of, a coupling transformerTl of the frequency translating unit FI'U' vover a back contact 4|" of the relay 22, and is transferred to the primary winding 42 of a coupling transformer- T3 of the translating unit FI'U 5 over the front contact 43 of relay 22 when that relay is energized. To be explicit, the normal plate circuitfor tube 21 can be traced from the positive terminal of a convenient source of current such as a battery 44 along wire 45, primary 10 winding 44 of transformer T4, back contact 4|, plate 28,.filament 33, biasing element 3435, aground connection 41, and thence by a ground connection to the negative terminal of battery 44. With the relay 22 picked up, this plate circuit extends from the positive terminal of battery 44 along wire 45, primary winding 42 of. transformer T3, wire 49, front contact 43, and thence as before traced to the negative terminal of battery 44. It follows that the output of ampliiier AMI is normally supplied to the coupling Ia'ansformer T4 of the frequency translating unit FTU, and is transferred tothe transformer T3 of the translating unit FI'U by the energizing of the relay 22. That is to say, in the normal 25 position of handle I9, frequencies of a given side band when picked up from the trafflc rails and supplied to the receiver AR are passed by the receiver AR to the transformer T2 and are ap-. plied to the grid of the tube 21; and hence are repeated in the transformer T4 of the translating unit FI'U with greatly-increased amplitude. The reverse position of handle I9 energizes the relay 22 and under this condition, voice variations developed by speaking into the microphone 35 M induce corresponding frequencies in the sec ondary of the transformer TI which are applied to the grid circuit of tube 21, and are repeated in the primary of the transformer T3 of the translating unit F'I'U greatly increased in amplitude. The frequency translating unit FTU includes a generator of carrier current, a push-pull balanced modulator circuit in which the carrier frequency is modulated at voice frequencies and then balanced out, a push-pull balanced demod- 45 ulator circuit in which the carrier current is added to a side band and then balanced out, a. band-pass filter in which the desired side band to be transmitted is selected, and a low-pass filter to pass the desired audio frequencies. This 5 frequency translating unit will now be described. The electron tubes and 5| are preferably standard three-element tubes, the filaments 52 of which are heated" by a battery 53. Normally the grid 55 of tube 50 is provided with a circuit 55 that extends from the grid 55 over. back contact 55 of relay 23, upper half of the secondary winding 51 of transformer T4, wire 53, resistor 59, biasing battery 53, and filament 52 of tube 50. Normally the grid circuit for the tube 5| extends from its grid 5| over back contact 62 of relay 23, lower half of secondary winding 51 of transformer T4, wire 58, resistor 59, battery 60, and At such time as relay 23 is energized and its front contacts 53 and 64 are closed, the grid 55 of tube 50 is connected to the upper half of the secondary winding 55 of transformer T3 over the front contact 63, and the grid 5| of tube 5| is connected to the lower half of the secondary winding 65 over the front contact 64 as will be readily understood by an inspection of the drawing. The secondary winding 51 of transformer T4 is one continuous winding with the wire 58 connected with the center turn; the secondary winding 55 of transformer T8 is similarly-wound. It 'is clear from the foregoing description of the grid circuits for tubes 66 and that alternating voltages induced in either the secondary winding 51 of transformer 5 T4, or in the secondary winding 65 of transformer T8 will cause the grids 65 and 6| to have unlike polarities. Y

The electron tube 86 is preferably of the threeelement type, and is provided with a plate cir- 10 cuit extending from the positive terminal of battery 44 along wire 45, inductor coil 61, plate 68, filament 69, and ground connections 18 and 48 back to battery 44. An oscillating circuit ll including an inductor l2 and a condenser 18, is 5 connectedwith the plate circuit df tube 66 through a blocking condenser "and to the grid of tube 66 through a biasing element I6, an intermediate terminal of inductor 12 being conmected to the filament 69 by a wire 11. The parts 20 of the oscillating circuit II are so proportioned and adjusted as to generate a carrier frequency current of say 7000 cycles per'second. It will be understood, of course, that other carrier frequencies could be selected if found desirable. The oscillating circuit H is coupledto the grid circuits of tubes 58 and 5| through a coupling condenser Ca and wire I8, the resistor 59 acting as a load resistance for the oscillating circuit. It is to be noted thatthe carrier voltages supplied 3 by oscillating circuit II to the grids 55 and 6| are in phase, whereas, the voltages supplied to the grids 55and 6| by the transformers T3 and T4 are opposite in phase.

At such time as relay 24 is deenergized and its back contacts 19 and 8|) are closed, the plates 8| and 82 of tubes 50 and 5|, respectively, are connected with the opposite terminals of the primary winding 83 of a transformer T6 in the conventional push-pull arrangement. The plate circuit for tube 59 extends from the positive terminal of battery 44 along wire 45, upper half of primary winding 83, back contact H9, plate 8|, filament 52, thence by ground connections 46 and 48 back to the battery 45; and the 45 plate circuit for the tube 5i includes positive terminal of battery 44, wire 45, the lower half of the primary winding 83, back contact 88, plate 82,"filament 52 of tube 5|, and ground connec-- tions 46 and 48 back to the battery 64. At such 50 time as relay 24 is energized and its front contacts 84 and 85 are closed, the plate circuit of tube 59 includes the upper half of the winding 88 of an autotransformer T5, .and front contact 84; and the plate circuit of tube 5i includes the lower half of winding 86 and front contact 85.

Condensers C1 and C2 are connected across the upper and lower halves of the secondary winding 85 of transformer T3 and serve as balancing condensers for balancing the carrier'voltages on the 60 grids 55 and 6| when transformer T3 is active during the transmitting of voice frequencies as will appear when the operation of the system is described. The transformer T4 is so proportioned as to pass the carrier side band frequencies, whereas, the transformer T3 is so proportioned as to pass the voice frequencies. Condensers C4 and C5 are connected across the two half windings of the primary 83 of transformer T6 "and function as balancing condensers to balance out the'carrier voltages passed to the plate circuits of tubes 50 and 5|, during the receiving of a telephone message.

The addition and subtraction of voltages and the voice frequency voltages supplied '5 by the transformer T8, when applied to the grids the carrier of the tubes' 58 and 6| produce a plate circuit current having the upper and lower side band frequencies of the carrier, and the carrier-is suppressed by the differential connection of the windings of transformer T6. The addition and subtraction of the carrier voltages and the side band frequency voltages supplied by the transformer T4 when applied to the grids of tubesifl and 5| demodulate the side band and produce in the, plate circuits audio frequencies. That is to 10 say, the voice frequencies supplied by the transformer T3 modulate the carrier current and the carrier is-balanced out leaving only the upper, and lower side bands. The side band voltage supplied through transformer T4 when combined 15 with the carrier is demodulated and the carrier balanced out leaving only audio frequencies and other products of demodulation. In other words,

in one condition, the voice frequencies are translated along a frequency scale to side bands of a 20 carrier frequency having a width equal to the voice frequency range; and a side band of the carrier having a width equal to the voice fre-. quency range is translated down the frequency scale to audio frequencies in the other condition. 25

Winding 86 of the auto-transformer T5 is included in the input of a band-pass filter BPF, and the secondary winding 81 of the transformer T8 is included inthe input of a low-pass filter LPF. The band-pass filter BPF may be any one 30 of several well known types and is so proportioned and adjusted as to eliminate one side bandof the carrier, and to transmit the other side band. The side band to be transmitted is supplied to the output circuit of 'the translating unit FIU which 35 is coupled to the input of a second stage amplifier AM2. The low-pass 'filter LPF may be any one of several well known types and is so proportioned and adjusted as to pass the audio frequencies, and to remove unwanted productions of deinodula- 40 tion. The audio frequencies passed by the filter LPF are supplied to the output circuit of the "translating unit FTU. As the specific type of neither the band-pass filter BPF, nor of the lowpassfilter LPF forms a part of our invention,' these devices are shown conventionally only for clarity. The output of the frequency translating unit FTU is coupled to the input of the amplifier AME overeither the front contact 88 of relay 25 or 750 over the back contact 89 of that relay. Under the normal deenergized condition ofv relay 25, the output of unit F'IU includes the low-pass filter LPF, back contact 89, condenser Co, a portion of winding 9|] of an auto-transformer Ti, condenser 55 C1, and wire 9|. With relay 25 energized, the output of unit FTU includes band-pass filter BPF, front contact 88, and thence as before traced. Electron tube 92 of amplifier AM2 is preferably of the three-element type having its 60 filament 93 heated by a battery 94. The grid 95 of tube 92 is provided with a circuit that includes the winding 9|] of transformer T1 and a biasing battery 98, a condenser 9'lbeing cohnected across the battery 96 to by-pass alternat- 5 mg voltages. When relay 26' is deenergized, the plate circuit for the tube 92 can be traced from the positive terminal of battery 44 along wire 98, primary winding 99 of a transformer T8, back contact I of relay 26, plate |9|, filament 93, 7 and thence by ground connections I82 and 48 back to the battery 44. With relay 26 picked up, this plate circuit includes battery 44, 'wire 98, primary winding I03 of a transformer T9, front contact I84, and then as before traced. 7

iii

v -The secondary winding I05 of transformer T8 is connected directly to a loud speaker LS. The two half sections of the secondary winding I 08 of the transformer T9 are connected to the grids I01 and I08 of tubes I09 and H0, respectively, of the transmitter AT, biasing batteries II I' and H2 being inserted in the connection as will be readily understood by an inspection of the drawing. It follows that the audio frequencies amplified by amplifier AM2 will be reproduced in the loud speaker, and that the side band frequencies aura... piified by amplifier AM2 are utilized to excite the grid elements of the tubes of the transmitter. The plates'II3 and ill of tubes I09 and H are, connected through the two half sections of the primary winding II5 of the output transformer TIO to the terminal B of .a convenient source of current in the conventional push-pull arrangement, the opposite terminal N of the source being connected to the filaments of the tubes. Hence, the side band frequencies applied to the grids of tubes I09 and H0 are supplied to the communicating circuit through transformer TIO at a relatively'high energy level.

In describing the operation of our system, we shall first assume that it is desired totransmit a telephone message. The operator wiEl first move the handle I9 to the right preparatory to removing the microphone from the rack, and in so doing the coils 2 and 3 are transferred to the output of the transmitter AT and the relays .22, 23, 24, 25 and 26 are all energized. The variations developed by speaking into the microphone .M induce corresponding frequencies in the secondary 3'! of transformer T1. These voice frequencies are applied to the grid of the amplifying tube 21 and are repeated in its plate circuit which includes the primary winding 42 of transformer T3 greatly increased in amplitude. The combination of the voice frequencies induced in the secondary 6.5 of transformer T3 with the carrier frequencies generated by the tube 66 and its related circuits produces a modulation of the carrier current. This addition and subtraction-of the voice voltages with the carrier voltages on the grids of tubes 59 and SI produce in the plate circuit of these tubes side band frequencies of the carrier, the carrier being suppressed or balanced out by the difi'erential connection of the windings of transformer T5. That is, the voice frequencies are translated along the frequency scale andnow appear as side bands of the carrier. The side band frequencies are applied to the band-pass filter BPF and one side band is eliminated and the other, preferably the upper side band,is-supplied to the output of the translating unit BTU which, in turn, is now coupled to the input of the second stage amplifier AM2. These side band frequencies which now represent the spoken message are applied to the input of the tube 92, and hence are repeated in the transformer T9 greatly increased in amplitude. Corresponding frequencies are induced in the secondary winding I08 of transformerT9 and are applied to the grids of tubes I09 and I I0 of the transmitter AT where they are amplified to a relatively high energy level and supplied to the traific rails I and I of. the communicating circuit. To sum up, the variations developed by speaking into the microphone are first amplified, then translated along the frequency scale to the upper side band frequencies of a carrier current, further amplified to a relatively high energy level, and finally delivered to the communicating circuit. a

Let us now assume that it is desired to receive a telephone message. The operator will replace the handle I9 to its normal position andtransfer the inductor coils 2 and I to the receiver AR, and deenergize the series of relays. The side band frequencies transmitted over the communis cating circuit from the. apparatus at the other location willbe picked up from the traific rails by the coils 2 and 3 and applied through the filter FI to the input of the amplifier AM. After being filtered and amplified at the receiver AR, the re- 10 ceived side band frequencies are applied to the 1 input of the tube 21 of the first stage amplifier AMI and repeated in its output greatly increased in amplitude. The side band voltages induced in the secondary 5Iof transformer Tl are combined with the carrier voltages generated by the tube 66 and its related circuits and applied to the grids of tubes 50 and SI. The addition and subtraction of the side band voltages with the carrier voltages demodulate the side band and cause to appear in the plate circuit of tubes 50 and SI audio frequencies and other producis of demodulation. The audio frequencies are passed to thelow-pass filter LPF where unwanted prod-- acts of demodulation are further eliminated, and the audio frequencies are then supplied to the input circuit of the amplifier AMI. The ampli-z fier AM2 repeats the audio frequency at the transformer T8, and hence they are delivered to the loud speaker LS which now reproducm the so telephone message. That is to say, theside band frequencies which represent the telephone mersage delivered to the communicating circuit at the second location are picked up from the communicating circuit, filtered, amplified, translated down the frequency scale to audio frequencies, further amplified, and finally delivered to the loud speaker.

Apparatus for a telephone such as here dis- .closed requires a small number of electron tubes 40 and auxiliary apparatus for the complete receiving and transmitting equipment. The use of the same generator of carrier current for both modulation and demodulation provides a frequency stability that insures ,both naturalness and intelligibility of the transmitted speech. The switching of the usual'audio amplifier of the receiving equipment into the transmitting equipment insures ample power for exciting the grids of the high energy level transmitter. The fila- 5o ments of all tubes normally heated for receiving eliminates the delay'required for heating up the tubes for transmission. This means that'as soon as the microphone is removed from its rack, the apparatus is instantly ready to transmit.

Although we have herein shown and described only one form of apparatus embmb'ing our invention, it is understood that various changes and modifications may be made therein within the scope-oi the appended claims without departing from the spirit and scope of our invention.

Having thus described our invention, what we claim is:

1. In combination with a frequency translating means including a pair of electron tubes and a source of a given carrier frequency current,

input circuits including the grids of said tubes and having a push-pull arrangement, output circults including the plates of said; tubes and having means adapted to pass a side band of said carrier frequency modulated with voice frequencies and including a receiver influenced by the communication circuit, a sending circuit includ- 5 ing a transmitter adapted to supply current to the communication circuit, a receiving circuit including a loud speaker, and manually controlled means for selectively coupling either the sending means and the sending circuit with said input and output circuits,respectively, or coupling the receiving means and the receiving circuit with said input and output circuits, respectively.

2. In combination with a frequency translating means including a pair of electron tubes and a source of a given carrier frequency current,

input circuits including the grids of said tubes and having a push-pull arrangement, output circuits including the plates of said tubes and having a push-pull arrangement, means for conzu necting said source with the grids of said tubes in parallel, sending means including a source of current and adapted to supply a low frequency signalling current, a communication circuit, receiving means adapted to pass a side band of 2.5 said carrier frequency modulated with a low fre-r quency signaling current and including a receiver influenced by the communication circuit, 9. sending circuit including a. transmitter adapted to supply current to the communication circuit, a receiving circuit including a signaling device responsive to low frequencies, and manually controlled means for selectively-coupling either the.

sending means and the sending circuit with said input and output circuits, respectively, or coupling the receiving means and the receiving circuit-with the input and output circuits, respectively. I

3. In combination; a frequency translating means including a pair of electron tubes eachhaving a plate, a grid and a filament; a. first and a second transformer each having a primary and a sendary winding, a relay for selectively connecting the grids to the opposite terminals of the secondary winding of either the first or the sec- 6nd transformer, means for connecting an intermediate terminal of each of said secondary windings with the filaments, a local source of carrier current connected with said intermediate terminals, plate circuits for said tubes having a pushw pull arrangement, sending means including a microphone adapted'to supply the primary winding of said first transformer with the voice frequencies of a spoken message, a communication circuit, receiving means efiectively influenced by said circuit and adapted to pass to the primary winding of said second transformer the side band frequencies of the carrier modulated with voice frequencies of a spoken message, and control means for governing said relay whereby the frequencies producedin the plate circuits are side band frequencies of the carrier modulated with the voice frequencies of the sending means, or

are audio frequencies corresponding to the demodulated side band frequencies of the receiv- 55 ing means. a

4. In combination with a frequency translating means including a pair of electron tubes each having a plate, a grid and a filament, and a generator of a given carrier frequency current;

70 plate circuits for said tubes having a push u arrangement, means for connecting said generator with the grids of said tubes in parallel, sending means including a microphone for supplying the voice frequencies of a spoken message,

76 a communication circuit, r ceiving means infiuenced by said circuit and adapted to pass the side band frequencies of said carrier frequency modulated with voice frequencies, a circuit controller, means including a contact of the controller for connecting the sending means with 5 the grids of said tubes in a push-pull arrangement, other means including a contact of the controller for connecting the receiving means with the grids of said tubes in a push-pull ar-. rangement, and a, manually controlled device for 10 selectively operating said controller whereby the voice frequencies of the sending meansare translated in the plate circuits to side band frequencies of the carrier or the side band frequencies passed by the receiving means are transl5= lated to audio frequencies.

, 5. In combination; a frequency translating means including a pair of electron tubes each having a plate, a grid and a filament; a first and a second transformer each having a primary 20 and a secondary winding, a first relay for selectively connecting thegrids with the opposite terminals of the secondary of either'the first or the second transformer, means for connecting an intermediate terminal of each of saidsec- 25 ondaries to the filaments, a local source of carrier current connected with said-intermediate terminals, a third and a fourth transformer each having a primary and a secondary winding, a

second relay for selectively connecting the plates 0' of said tubes in a push-pull arrangement with the primary of either the third or thefourth transformer, sending means including a microphone for supplying the primary'of the first transformer with voice frequencies of a spoken message, receiving means including a receiver for supplying the primary of the second transformer with side band frequencies of a carrier modulated by the voice frequencies of a spoken -message, a sending circuit connected with the 40 secondary of the third transformer including a transmitter, a receiving circuit connected with the secondary of the fourth transformer including a loudspeaker, and manually controlled means for governing said first and second relay 45 and so arranged that the voice frequencies of the sending means are translated to side band frequencies of the local carrier in the sending circuit and the side band frequencies of the receiving means are translated to audio frequencies in the receiving circuit.

6. A telephone system comprising, a communication circuit, a receiver, a transmitter, a circuit controller adapted to couple the receiver or the transmitter with said circuit, a microphone, 5.

a loud speaker; a frequency translating means including a pair of electron tubes connected in a push-pull arrangement, and a generator of carrier current so coupled with the input circuits of said tubes as to'be balanced out in the 69 output circuits of said tubes, a first coupling means for connecting either the receiver or the microphone with the input circuits of the tubes of said translating means, a second coupling means for connecting the output circuits of the 1 tubes of the translating means to either the transmitter or to the loud speaker, and manually controlled means for governing said circuit controller and said first and second coupling means,

and arranged in such a manner that voice frequencies produced by speaking into the microphone are delivered to,the communication circuit as side band frequencies of the carrier, and side band frequencies of a carrier modulated at voice frequencies received from the communica- 7 5,

' livered to the loud speaker as corresponding audio frequencies.

" cation circuit, a receiver, a transmitter, a circuit with the output circuits of the tubes of the" translating means, and manually controlled means for governing the circuit controller and said first and second relay, and arranged in such a mannerthat voice frequencies produced by speaking into the microphone are delivered to the communication circuit as side band frequencies of the carrier, and side band frequencies of a carrier modulated at voice frequencies received from said communication circuit are de- 8. A telephone system comprising, a communication circuit, a receiver, a transmitter, a circuit controller adapted to couple the receiver or the transmitter to said circuit, a microphone, a telephone receiver, a first stage amplifier, a frequency translating means capable of translating voice frequencies to side band frequencies of a carrier or of translating side band-frequencies of the carrier to audio frequencies, a. second stage amplifier, a first coupling means for connecting the receiver or the microphone to the input of the first stageamplifier, a second coupling means for connecting the output of the first stage amplifier to the input of the translating means, a third coupling means for connecting the output of they translating means to the input of the second stage amplifier, a fourth coupling means for connecting the output of the second stage amplifier to either the telephone receiver or to the transmitter; and manually controlled means to govern said circuit controller and said first, second, third and further coupling means and arranged in such a manner that voice frequencies produced by the microphone are delivxered to the communication circuit as amplified side band frequencies of the carrier or side band frequencies received from said circuit are delivered to the telephone receiver as amplified audio frequencies.

9. A telephone system comprising, a communicating circuit, a receiver, a transmitter, a circult controller adapted to couple the receiveror the transmitter to said communicating circuit,

a microphone, a telephone receiver, a first stage amplifier; a frequency translating means. including a generator of carrier current, and a pair of electrontubes placed in a push-pull arrangement with the carrier voltage applied to both grids in phase; a band-pass filter, a low-pass filter, a second stage amplifier, afirst coupling means for. connecting the receiver or the microphone with the input of the first stage amplifier, a. second coupling means for connecting the out put of the first stage amplifier to the input of the translating means in such a manner that the voltages applied to the two grids are opposite in phase, a third coupling means for conaoaavsa f necting the'output of the translating means to the band-pass filter or to the low-pass filter, a fourth coupling means for connecting the bandpass filter or the low-pass filter with the input of the second stage amplifier, a fifth coupling means for connecting the output of the second stage amplifier with either the telephone receiver or with the transmitter; andmanually controlled means to govern said circuit controller and said first,lsecond, third, fourth and fifth coupling-means and arranged in such a manner that voice frequencies developed in the microphone are delivered to the communicating circluits as amplified side band frequencies of the carrier or side band frequencies received from said circuit are delivered to the telephone receiver as amplified audio frequencies.

10. A telephone system comprising, a communication circuit, a sending means including a microphone for supplying a voice frequency current, receiving means coupled with said circuit and adapted to pass the side band frequencies of a carrier frequency modulated with voice frequencies, a frequency translating means ineluding a pair of electron tubes having a pushpull arrangement of plate circuits and a generator of said carrier frequency current so connected with the grids of the tubes that the carrier is applied to the grids in phase, circuit controlling means, means including a;contact of the circuit controlling means for coupling the sending means with the frequency translating means and arranged that the voice frequency current is applied to the grids of said tubes opposite in phase, means including a contact of the circuit controlling means for coupling the receiving means with the frequency translating means and arranged that the side band frequencies are applied to the two grids opposite in phase, a receiving circuit including a loud speaker, a sending circuit including a transmitter capable ofasupplying current to the communication circuit, .means including a contact of the circuit controlling means for coupling the receiving circuit with said plate circuits, other means including a contact of the circuit controlling means for coupling said sending circuit with said plate circuits, and a manually controlled device for selectively operating said circuit controlling means, whereby voice frequencies produced in the microphone are transmitted'to the communication circuit as side band frequencies of the carrier or the side band frequencies received from the communication circuit are effective to operate the loud speaker.

. 11. In combination, a communication circuit, a source of periodic signaling current, a receiver AR coupled with said circuit effectively infiuenced by periodic signaling current, an amplifier including an electron tube, a signaling device LS responsive to periodic signaling current, a transmitter AT coupled with the communication circuit for supplying current thereto, circuit controlling means, means including a first set of contacts of said circuit controlling means for coupling said periodic current source and said transmitter with the grid and plate circuits of the tube. of the amplifier respectively, other means including a second set of contacts of the circuit controlling means for coupling said reand plate circuits of said tube respectively, and manually controlled means toy operate said circuit controlling means for selectively closing @said first and second set contacts.

12. In combination, a communication circuit extending from a location, a source of periodic signaling current and a transmitter AT coupled with said circuit at said location, a signaling *device LS responsive to periodic signaling curtube for picking up such signaling current from said circuit, and means to operate said control means for selectively closing said first and second set of contacts.

134A telephone system comprising, a communication circuit, sending means including a source of current and a telephone transmitter, a. sending circuit including means coupled with the communication circuit, receiving means coupled with said communication circuit, a receiving circuit including a current source and a telephone receiver, an amplifier including an electron tube having a grid and a plate, control means including a plurality of circuit controlling contacts, means including a first and a second contact of the control means for coupling said sending means and said sending circuit with the grid and plate of the tube respectively for supplying a telephone current to the communication circuit, other means including a third and a fourth contact of the control means for coupling said receiving means and said receiving circuit with the grid and plate of the tube'respectively for picking up a telephone current from the communication circuit, and manually operated means for governing the control means to selectively close either;

said first and second contacts or said third and fourth contacts.

14. A telephone system comprising, a communication circuit, an inductor coupled with said circuit, sending means including a. current source and a microphone, a receiving circuit including a loud speaker, an amplifier including an electron tube having a grid and a plate, control means including a plurality of circuit controlling contacts, means including a first and a second contact of said control means for connecting said inductor and said loud speaker with the grid and plate of said tube respectively for picking up a telephone current from the communication circuit, other means including a third and a fourth contact of the control means for connecting said sending means and said inductor with the grid and plate of said tube respectively for supplying a telephone current to the communication circuit, and manually operated means for governing the control means to selectively close either said first and second contacts or said third and fourth contacts. 15. In combination, a communication circuit,

an inductor coupled with said circuit, an amplifier including an electron tube, a plate and a grid circuit for said tube, control means including a plurality of contacts, means including a first contact of said control means for coupling said inductor with the grid circuit of said tube for amplifying current picked up from the communication circuit, other means including a second contact of the control means for coupling said inductor with the plate circuit of said tube for supplying ourrent amplified by said tube to said communication circuit, and means for governing said control means to selectively close said first. and second contacts.

16. A telephone system comprising, a communication circuit, sending means including-a microphone, receiving means including a loud speaker, an amplifier including an electron tube, a plate and a grid circuit for said tube, control means including a plurality of circuit controlling 10 contacts, means including a first and a second contact of the control means for coupling the grid and plate circuits of said tube with the communication circuit and said receiving means respectively for amplifying and reproducing a tele- 15 phone current picked up from the communication circuit, other means including a. third and a fourth contact of the control means for coupling the grid and plate circuits of the tube with the sending means and the communication circuit 20 respectively for amplifying and supplying a telephone current to said circuit, and manually operated means for governing said control means.

1'7. In-combination, a,cor nmunication circuit extending from a location, a source of periodic 5 signaling current and a signaling device responsive to periodic signaling current at said location, an amplifier including an electron tube having a grid element and a plate element, circuit controlling means capable of assuming a. first and 30 a second condition, means governed by said circuit controlling means and including a first and a second contact each closed in the first condition to effectively couple the communication circuit and the signaling device to the grid and 35 plate elements of the tube respectively, other means governed by said circuit controlling means and including a third and a. fourth contact each closed in the second condition to eifectively couple the source of periodic current and the com- 40 munication circuit to the grid and plate elements of the tube respectively, and manually operable means effective to selectively establish the first and second conditions of said circuit controlling means. 15

18. In combination with a frequency translating means including a pair of electron tubes which are provided with push-pull grid circuits and with push-pull plate circuits together with -a source of carrier frequency current connected 5 first and a second contact each closed in the first condition to effectively couple the communication circuit and said signaling device to the 0 grid circuits and plate circuits of said electron tubes respectively, other means governed by said circuit controlling means and including a third and a fourth contact each closed in the second condition to effectively couple the source of pe- 5 riodic signaling current and the communication circuit to the grid. circuits and plate circuits of said tubes respectively, and manually operable means effective to selectively establish the first and second conditions of said circuit con- 7 trolling means. 7

19. In combination, a frequency translating means including a source of carrier frequency current and a first and a second circuit network,

said first circuit network operative to modulate the carrier witha periodic signaling current supplied to its input terminals and to deliver a side band of such modulated carrier to its output terminals, said second circuit network operative to mix such side band supplied to its input terminals with said carrier and to deliver only the periodic signaling frequencies to its output terminals, 9. communication circuit, a source of such periodic signaling current; a signaling device responsive to such signaling current, circuit controlling means capable of assuming a first and a second condition, means governed by said circuit controlling means and including a first and a second contact closed in the first condition for effectively connecting the communication circuit and thesignaling device with the input and output terminals of said second network respectively, other means governed by said circuit controlling means and including a third and a fourth contact closed in the second com 5 dition for effectively connecting the source oi pe-' riodic signaling current and the; communication circuit with the input and output terminals of said first network respectively, and manually operable means eilective to selectively establish the 10 first and second condition of said circuit controlling means. i

' ANDREW J. SORENSEN.

LELAND D. WHITEIOCK. 

